(1) Field of the Invention
This invention relates to a film for a pellicle which is used as a dust cover for a photomask or reticle (which is hereinafter referred to as "mask" for brevity) used at the lithographic step in the course for the production of a semiconductor integrated circuit.
The film for a pellicle of the present invention can also be used in lithography utilizing as a light source i-line having a wavelength of 365 nm of a mercury arc light, which is recently used.
(2) Description of the Related Art
In the production of a semiconductor integrated circuit, the step of patterning by light exposure a semiconductor wafer coated with a resist material is important because it dominates the yield of the integrated circuit. If scratches or dusts are present on a mask which is an original sheet for patterning, the images of the scratches or dusts as well as those of the intended pattern are transferred onto the wafer and consequently shortcircuiting or wiring breakage occasionally occurs in the integrated circuit. Therefore, protection of the mask is important for enhancing the yield of the integrated circuit and a pellicle is now often used as a dust cover for protection of the mask.
As pellicle film-forming materials, nitrocellulose, cellulose acetate, cellulose propionate, polyethylene terephthalate, polypropylene and polymethyl methacrylate have heretofore been proposed. Of these, nitrocellulose is used as a pellicle film in a light exposure apparatus utilizing g-line (wavelength: 436 nm) of a mercury arc light as the light source.
Recently, the demand for high densification of a semiconductor integrated circuit requires shortening of the wavelength in lithography light source, and thus, the i-line having a wavelength shorter than that of the g-line is used as the lithography light source, as well as the conventional g-line.
Conventional pellicle film-forming materials such as nitrocellulose can be used in the lithography using as the light source near ultraviolet rays such as g-line and h-line of a mercury arc light, but cannot advantageously used in a light exposure apparatus using i-line of a mercury arc light, as the lithography light source. This is because the conventional pellicle film-forming materials exhibit a light absorption in the deep ultraviolet region, and therefore, rapidly become opaque or are deteriorated when exposed to the i-line. Therefore, a pellicle film is now eagerly desired which can advantageously be used for i-line as well as g-line or h-line of a mercury arc light.
The following are usually required for a film for a pellicle: (1) the pellicle film does not exhibit a light absorption, i.e., is transparent, in the wavelength region of the light source of the light exposure apparatus, and (2) the pellicle film is durable, that is, even when the pellicle is continuously exposed to light for a long time, it does not become opaque nor is deteriorated. When the pellicle film is used in a light exposure apparatus using i-line of a mercury arc light as the light source, it is required that the pellicle film is transparent to and durable against the i-line.
As a film for a pellicle used in a light exposure apparatus utilizing the i-line as the light source, a pellicle film having a multilayer structure has been proposed which is comprised of a nitrocellulose film having reflection-preventing layers composed of a fluorine-containing polymer on both surfaces thereof (for example, see Japanese Unexamined Patent Publication No. H1-100549). The process for preparing this pellicle film is complicated, the yield is low and the production is costly.
As another pellicle film used in a light exposure apparatus utilizing i-line of a mercury arc light, a pellicle film comprised of a polyvinyl acetal resin has been proposed (for example, see Japanese Unexamined Patent Publication No. H1-172430). The polyvinyl acetal resin pellicle film can be used without reflection-preventing layers and the production thereof is neither complicated nor costly, but this pellicle film is not durable, that is, when exposed to the i-line for a long period, it becomes opaque and is deteriorated.